Certain prior art procedures for measuring low fluid flow rates are described in the Benson et al. paper "Thermal Mass Flowmeter" (Industrial and Control Systems, February, 1970, p. 85) and in the Hawk et al. article "Measuring Small Gas Flows into Vacuum Systems" (The Journal of Vacuum Science and Technology, Volume 6, No. 1, p. 255). The Hawk et al. article makes reference to classic procedures for measuring small gas flow rates, e.g. soap bubbles. However, basically both of these references relate to thermal mass flowmeters the operation of which is based upon relating the mass flow of a gas to a heat absorption process.
It would be of considerable advantage to the art to have available a low impedance, low flow-rate meter operating passively, i.e. not requiring power.